In normal TWiki operations as a web server this routine is called by the
BEGIN block of TWiki.pm. However, when benchmarking/debugging it can be
replaced by custom code which sets the configuration hash. To prevent us from
overriding the custom code again, we use an "unconfigurable" key
$cfg{ConfigurationFinished} as an indicator.

Note that this method is called by TWiki and configure, and only reads
TWiki.spec= to get defaults. Other spec files (those for extensions) are
not read.

The assumption is that configure will be run when an extension is installed,
and that will add the config values to LocalSite.cfg, so no defaults are
needed. TWiki.spec is still read because so much of the core code doesn't
provide defaults, and it would be silly to have them in two places anyway.